Magnetic structure for a loudspeaker



April 19, 1966 UNN|NGHAM 3,247,331

MAGNETIC STRUCTURE FOR A LOUDSPEAKER Filed July so, 1954 ifi J 'QQQKQ Y\\Q \QQ\\ I INVENTOR.

DAVID H. CUNNINGHAM ATTORNEY 3,247,331 I a tented Apr. 19, 1966 3 247 331 MAGNETIC sTRUcTIJREFoR A LOUDSPEAKER David H. Cunningham, Haddonfield, N .J., assignor to Radio Corporation of America, a corporation of Delaware Filed July 30, 1954, Ser. No. 446,871 1 Claim. (Cl. 179115.5)

This invention relates to magnetic structures, and more particularly to permanent magnetic structures for use with dynamic loudspeakers.

of another embodiment of a magnetic structure associated with a loudspeaker, in accordance with the present invention.

Referring particularly to FIGURE 1, a magnetic structure 8 includes an annular permanent magnet 10 forming an outer pole piece for the magnetic structure. The magnet is welded or otherwise suitably connected to a cup- In a dynamic loudspeaker having a voice coil suspended within an air gap, the only useful lines of magnetic flux provided by a magnetic circuit are those which pass through the voice coil. The sensitivity of the loudspeaker will increase in accordance with the number of lines of magnetic flux passing through the voice coil.

During the operation of a loudspeaker, the force on the voice coil may be determined by the formula:

2- 1 F- 1110 dynes where o is the number of lines of flux at the voice coil, A is the area of the voice coil winding in square centimeters, i is the number of amperes of current in the coil, and l is the length of the conductive winding in centimeters. Hence, for a given size magnet, it is important that the be as high as possible to obtain high performance at low cost.

A major cause of inefficiency within a magnetic structure used in a dynamic loudspeaker is the amount of leakage flux which does not pass through the voice coil in the air gap and consequently is wasted. The magnitude of this leakage flux depends upon, among other things, the geometry of the magnetic structure.

In conventional magnetic structures used with dynamic loudspeakers, the disposition of the magnet with respect to the air gap is such that a 90 relationship exists between the direction of the magnetic flux in the magnet and the direction of the magnetic flux within the air gap associated with the magnetic structure. This arrangement, while satisfactory in many respects, permits a relatively high amount of leakage flux to exist within the magnetic structure and results in only a portion of the magnetic lines of flux from the magnet reaching the air gap to be usefully employed.

It is an object of this invention to provide an improved magnetic structure in which the amount of leakage flux is reduced.

It is a further object of this invention to provide an improved magnetic structure in association with a dynamic loudspeaker in which the magnetic lines of flux passing through a voice coil suspended in an air gap is greatly increased.

The present invention provides a magnetic structure for use with a dynamic loudspeaker. An air gap, adapted to receive a suspended voice coil is formed by a pair of pole pieces. A permanent magnet produces the magnetic lines of flux and forms one of the pole pieces. The magnet is disposed adjacent the air gap to produce lines of flux within the air gap which are in substantial alignment with the magnetic lines of flux within the magnet.

Other objects and advantages of the present invention will become apparent and suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specification in connection with the drawing, in which:

FIGURE 1 is a cross-sectional view, partly broken away, of a magnetic structure associated with a loudspeaker, in accordance with the present invention, and

FIGURE 2 is a cross-sectional view, partly broken away,

shaped yoke member 1G. The magnet may be made of ceramic, Alnico or any other permanent magnet material. The yoke member may be ferromagnetic material such as cold rolled steel or other suitable material. An inner pole piece 14 made of material similar to the yoke is centrally attached to the yoke member 12 as shown. The magnet 10 is disposed with relation to the inner pole piece to provide an air gap 16. The end of the magnet facing the air gap is tapered to improve the magnetic efliciency of a structure.

A suspension disk 18 connects a voice coil form 26 to a frame member 22 (partly broken away.) A voice coil 24 is wound upon the voice coil form 26 and is adapted to receive electric-a1 signals. The cone diaphragm 20 is connected to the voice coil form 26. The voice coil is centrally suspended within the air gap. Electrical signals applied to the voice coil cause the voice coil to move in an axial direction within the magnetic field. This causes the diaphragm to vibrate in a well-known manner, thereby translating the electrical signals into corresponding acoustical signals.

It is seen that the magnetic circuit in this embodiment of the present invention includes the magnet 10, the yoke member 12, the air gap 16, and the inner pole piece 14. Also located within this magnetic circuit at the air gap is the voice coil 24. The direction of the magnetic lines of flux within the magnetic circuit is as indicated by the arrows in the drawing. It is seen that the magnetic lines of flux within the air gap are in substantial alignment with the magnetic lines of flux within the magnet, and do not bend degrees in between these two members as in many conventional magnetic structures. Increased magnetic ef tficiency is attained since the magnet is placed in an optimum oriented position relative to the air gap, thereby minimizing the amount of leakage flux within the structure.

It is further seen that the magnet 10 is disposed adjacent the air gap 16. This disposition of the magnet further reduces the leakage losses within the structure and increases the efliciency of the magnetic circuit. It may be desirable to plate the ends of the magnet facing the air gap with steel or other permeable material. However, for maximum efiiciency, it is desirable to have the magnets disposed as close to the air gap as practical to minimize the areas of the pole pieces.

Although the yoke member 12 is shown as having a cup shape, it is understood that a conventional U shaped rectangular plate may also be used. In that case, a rectangular magnet having a central opening would be employed. The inner pole piece would be inserted into the central opening to form an air gap with the magnet and the voice coil suitably suspended within the air gap. The use of cup or U shaped yoke members associated with loudspeakers is well known in the art.

Referring particularly to FIGURE 2, a magnetic structure 2-8 includes a cup shaped yoke member 30. An annular ring 32 forms an outer pole piece for the magnetic structure and is welded or otherwise suit-ably attached to the yoke member. A magnet 36 is attached to the end of the pole stern member 34.

The annular ring or pole piece 32 and the magnet 36 are disposed to provide an air gap 38 therebetween. A loudspeaker cone diaphragm 40 (partly broken away) is suitably connected to a voice coil form 46 and is supported at its outer periphery by a frame member 42 (partly broken away). A corrugated suspension ring 44 supports the voice coil and cone apex by anchorage at its outer rim to the frame member 42. The voice coil form 46 having a voice coil 48 wound thereon is attached to the diaphragm and suspended within the air gap between the magnet and the outer pole piece. The voice coil winding is adapted to receive electrical signals to cause vibrations of the diaphragm in a well-known manner.

The directions of the magnetic lines of flux within the magnetic structure is as indicated by the arrows in the drawing. It is seen that the magnetic lines of flux within the magnet are in substantial alignment with magnetic lines of flux within the air gap; Thus, the magnet is so oriented that no bending of the magnetic lines of flux between the magnet and the air gap takes place. This an rangement greatly increases the efficiency of the magnetic circuit associated with the'magneti'c structure since the leakage flux is minimized.

Locating the magnet adjacent the air gap also helps to minimize the leakage loss, to provide still greater eificiency in the magnetic circuit associated with the magnetic structure and air gap. 7

As was the case in connection with FIGURE 1, the yoke member 30 may be a rectangular plate bent into a- U shape rather than the cup shape, as described.

What is claimed is:

A transducer comprising an annular disk, permanently magnetized radially with one pole distributed about its inner periphery and the other distributed about its outer periphery; a yoke of magnetic material comprising an in ner cylinder having an outer diameter smaller than the inner diameter of the disk, an outer cylinder concentric with said inner cylinder and having an inner diameter of 4, a size to receive tightly the outer periphery of the disk, and a Web connecting said inner and outer cylinders at one end; said disk being received in said yoke with its outer periphery engaging said outer cylinder and its inner periphery spaced from the inner cylinder and cooperating therewith to define an air gap, a coil received in said air gap, and means including a diaphragm supporting said coil in said gap for vibratory movement therewith in said magnetic field.

References Cited by the Examiner UNITED STATES PATENTS 1,732,029 10/19 2 9 Round 179-4155 1,875,004 8/19 3 2 Horsburg'h 179117 2,029,282 '1/1936 Serge 179-417 2,084,945 1/;19 37 Cornwell 17 9- 1155 2,175,994 10/1939 Quarn 179-,- -117 2,187,236 1 /1940 'Hansdorf 179 117 2,590,935 4/ 1195 2 Charles'worth 1791'15.5 2,751,512 6/1956 een et a1. 179 '117 2,864,155 12/1953 Wil1s on -179 -11s.5 2,978,671 4/1951 Harris 17911-5.5

FOREIGN PATENTS 498,215 5/ 1930 Germany.

549,446 4/ 1932 Germany.

403,778 1/1934 Great Britain.

0 ROBERT H. ROSE, Primary Examiner.

R. R. SCI-ILBM-MER, H. W. GARNER, F. N. CARTEN,

Assistant Examiners. 

